1. Field of the Invention
The present invention relates to a method for manufacturing a rubber product and the rubber product obtained by same method.
2. Description of the Art
Conventionally, automotive radiator hoses and other rubber products have been made from a rubber composition containing rubber, such as an ethylene-propylene-diene terpolymer (EPDM), a reinforcing agent, such as carbon black, a vulcanizing agent, such as sulfur, and a plasticizer, such as oil. The rubber composition is typically prepared through a mastication process and a kneading process. In the mastication process, the rubber is plasticized by the application of mechanical shearing force to disentangle aggregated molecules or the cleavage of molecular chains. The plasticized rubber is mixed with carbon black, sulfur, oil, and other materials in the kneading process. For further details, see Japanese Unexamined Patent Publication No. 10-180941 and “Gomu no Jiten” (an encyclopedia of rubber), edited by Michio Okuyama et al., Asakura-shoten, Tokyo (2000), p. 303.
However, low-polarity rubber (rubber having few polar groups), such as EPDM, has a low affinity for carbon black. Thus, it is difficult to disperse the carbon black uniformly in the rubber polymer. Furthermore, when high-polarity carbon black is used to increase the reinforcing property (strength) of a rubber product, the affinity between the low-polarity rubber and the carbon black further decreases.
To achieve sufficient mixing in the above-mentioned preparation, various procedures, including a masterbatch method, have been proposed. However, even using these mixing procedures, carbon black may separate from the rubber polymer and reaggregate over time. In particular, the high-polarity carbon black or carbon black having a minute particle diameter (minuter than FEF (Fast Extruding Furnace) carbon black), tends to separate from the rubber polymer and may reaggregate. The reaggregation results in surface roughness of a final rubber product, and also leads to reduction or variation in the electrical resistance of the rubber product.
On the other hand, the addition of a silane coupling agent, sulfur or a sulfur-based vulcanization accelerator, and/or a phenol-based stabilizer to the rubber composition in preparation thereof is proposed to prevent the separation between the carbon black and the rubber polymer and the reaggregation of the carbon black. However, the silane coupling agent is expensive and increases the production cost, which cannot satisfy the demand for lowering cost of a rubber product. In addition, since unpleasant odor of the silane coupling agent deteriorates the work environment, there is a need for nonuse of the silane coupling agent. The silane coupling agent may also deteriorate the moldability of extrusion molding (or increase the surface roughness of an extrudate). For sulfur or the sulfur-based vulcanization accelerator, temperature control in kneading is required (temperature control of 150° C. or lower). Especially, where general-purpose compounds are subjected to short-time mixing, they are often kneaded at temperatures higher than 150° C. In such a case, sulfur or the sulfur-based vulcanization accelerator tends to have little dispersion effect on the carbon black. While the phenol-based stabilizer, such as those manufactured by Sumitomo Chemical Co. Ltd. under the trade name of SUMILIZER GM or SUMILIZER GS, is suitably used owing to its excellent dispersion effect on the carbon black, it is very expensive and is not-suitable for general use.